There are extensive deposits of viscous hydrocarbons throughout the globe, including large deposits in the Northern Alberta tar sands, which are not recoverable with traditional oil well production technologies because the hydrocarbons are too viscous to flow. Indeed, the viscosity can be as high as one million centipoise. In some cases, these deposits are mined using open-pit mining techniques to extract the hydrocarbon-bearing material for later processing to extract the hydrocarbons. However, many sites are not amendable to open-pit mining techniques.
As an alternative methodology, thermal techniques can be used to heat the reservoir fluids and rock to reduce hydrocarbon viscosity and thus produce the heated, mobilized hydrocarbons from wells. One early technique for utilizing a single well for injecting heated fluids and producing hydrocarbons is described in U.S. Pat. No. 4,116,275, which also describes some of the problems associated with the production of mobilized viscous hydrocarbons from horizontal wells.
One important advance in the thermal recovery of viscous hydrocarbons is known as steam-assisted gravity drainage (SAGD) process. The SAGD process is currently the only commercial process that allows for the extraction of bitumen at depths too deep to be strip-mined. For example, the estimated amount of bitumen that is available to be extracted via SAGD constitutes approximately 80% of the 1.3 trillion barrels of bitumen in place in the Athabasca oil-sands in Alberta, Canada. Various embodiments of the SAGD process are described in CA1304287 and corresponding U.S. Pat. No. 4,344,485.
In the SAGD process, two vertically spaced horizontal wells are used to inject steam and collect the oil. Steam is pumped through an upper, horizontal injection well into a viscous hydrocarbon reservoir while the heated, mobilized hydrocarbons are produced from a lower, parallel, horizontal production well vertically spaced a few meters proximate to the injection well. Both the injection and production wells are typically located close to the bottom of the hydrocarbon deposits.
The SAGD process is believed to work as follows. The injected steam creates a “steam chamber” in the reservoir around and above the horizontal injection well. As the steam chamber expands upwardly and laterally from the injection well, viscous hydrocarbons in the reservoir are heated and mobilized, especially at the margins of the steam chamber where the steam condenses and heats a layer of viscous hydrocarbons by thermal conduction. The heated, mobilized hydrocarbons (and steam condensate) drain under the effects of gravity towards the bottom of the steam chamber, where the production well is located. The mobilized hydrocarbons are then collected and produced from the production well.
The rate of steam injection and the rate of hydrocarbon production may be modulated to control the growth of the steam chamber to ensure that the production well remains located at the bottom of the steam chamber and in a position to collect the mobilized hydrocarbons.
In order to initiate a SAGD production, thermal communication must be established between an injection and a production SAGD well pair. Initially, the steam injected into the injection well of the SAGD well pair will not have any effect on the production well until at least some thermal communication is established because the hydrocarbon deposits are so viscous and have little mobility. Accordingly, a start-up phase is required for the SAGD operation.
Typically, the start-up phase takes about three months before thermal communication is established between the SAGD well pair, depending on the formation lithology and the actual inter-well spacing. The traditional approach to starting-up the SAGD process is to simultaneously operate the injection and production wells independently of one another to circulate steam. The injection and production wells are each completed with a screened (porous) casing (or liner) and an internal tubing string extending to the end of the liner, forming an annulus between the tubing string and casing. High pressure steam is simultaneously injected through the tubing string of both the injection and production wells. Fluid is simultaneously produced from each of the injection and production wells through the annulus between the tubing string and the casing.
In effect, heated fluid is independently circulated in each of the injection and production wells during the start-up phase, heating the hydrocarbon formation around each well by thermal conduction. Independent circulation of the wells is continued until efficient thermal communication between the wells is established. In this way, an increase in the fluid transmissibility through the inter-well span between the injection and production wells is established by conductive heating.
The pre-heating stage typically takes about three to four months. Once sufficient thermal communication is established between the injection wells, the upper, injection well is dedicated to steam injection and the lower, production well is dedicated to fluid production.
A variant of SAGD is expanded solvent steam-assisted gravity drainage (ES-SAGD). In ES-SAGD a solvent is used in conjunction with steam from water. The solvent then evaporates and condenses at the same condition as the water phase. By selecting the solvent in this matter, the solvent will condense with the condensed steam, at the boundary of the steam chamber. Condensed solvent around the interface of the steam chamber dilutes the oil and in conjunction with the heat, further reduces its viscosity.
Both SAGD and ES-SAGD require the use of water to be injected down-hole. Due to costs and environmental concerns, the use of water for the production of heavy oil can be technically challenging. Furthermore, as in all thermal recovery processes, the cost of steam generation is a major part of the cost of oil production. Historically, natural gas has been used as a fuel for Canadian oil sands projects, due to the presence of large stranded gas reserves in the oil sands area, but this resource is getting more expensive and there are competing demands for the natural gas. Other sources of generating heat are under consideration, notably gasification of the heavy fractions of the produced bitumen to produce syngas, using the nearby (and massive) deposits of coal, or even building nuclear reactors to produce the heat. All of these contribute to cost.
In addition to the large operating costs of generating steam, a source of large amounts of fresh and/or brackish water and large water re-cycling facilities are required in order to create the steam for the SAGD process. Thus, lack of water and competing demands for water may also be a constraint on development of SAGD use.
Thus, what is needed in the art are improvements to oil recovery techniques that further improve cost effectiveness and/or decrease the environmental impact.